Thousands of homes have been evacuated in the US states of Idaho and Nevada after a wildfire grew by 200 sq miles (518 sq km) in 24 hours.
Two large fires combined to create the blaze, which officials said was only a mile from a US Air Force training base.
No-one has been hurt but the homes of some 7,500 people are at risk, a fire officer told the Associated Press.
Hot weather and dry conditions in the western US have fuelled a series of fierce wildfires in recent weeks.
Fire information officer Chuck Dickson said the fire along the border of Idaho with Nevada could damage radar and tracking facilities at Mountain Home Air Force Base.
"When a fire moves as quickly as this one does, pushed by the wind, it creates a very hot heat front," he told AP.
"That will impact even steel structures quite easily."
The wildfire, which had spread to cover 880 sq miles (2,280 sq km) by Sunday, has burned large areas of grazing land and may have killed cattle in its path, he said.
Firefighters in the western states of Oregon and Utah were also battling to contain fires over the weekend, fire officials said.
In California, firefighters lifted the last of the evacuation orders imposed in northern Santa Barbara County, where a fire has been burning in the Los Padres National Forest for weeks.
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